1. Field of the Invention
The present invention relates to a touch sensor for a probe, which, for example, is used when measuring micro-configuration of the surface of a test piece by equipment such as a micro-configuration measuring device and a surface roughness measuring device or inner configuration of a hole by a small hole measuring device.
2. Description of the Related Art
Conventionally, micro-configuration measuring devices are used when examining a test piece for research and development purposes as well as for production activities in the fields of precision machining or semiconductor manufacturing. The device measures micro-dimensions such as a surface roughness or a step on the machined surface and a thickness of a thin film by a vertically oscillating a stylus, which is brought in contact with and moved about the test piece. The change in the vertical oscillation of the stylus is then converted into an electrical signal to be read.
One example of such a mechanism involving a stylus used in micro-configuration measuring devices as described above is a touch sensor, which is disclosed in Japanese Patent Laid Open No. 2001-91206.
In FIG. 6, this touch sensor 10 includes a stylus holder 11, a stylus 12 which is held by arms 13, 14, 15 and 16 and has a tip 12A making contact with the test piece, and a couple of piezoelectric elements 19, one of which is attached to the stylus on one side and the other of which on the side opposite thereto. Each piezoelectric element 19 is made of two parts, one being an oscillation means 17 and the other a detection means 18, joining at the center.
Given such a structure, if an electrically alternating signal of an appropriate oscillation frequency is applied to the oscillation means 17, then the stylus 12 starts oscillating in a resonating manner in the axial direction. If, in this resonating state, the tip 12A of stylus 12 makes contact with the test piece, then the resonating state changes, and this change of state can be detected by monitoring output from the detection means 18.
In precision measurement where micro-configuration is measured by using a touch sensor described above, it is important that measuring force acting between a test piece and the tip of a stylus be controlled below a prescribed value; the test piece and the tip not be damaged; and movement of the stylus tip accurately reflect the surface configuration of the test piece. Accordingly, a probe, which is equipped with a mechanism of controlling the measuring force below a prescribed value, is available.
One example is a probe for a micro-configuration measuring device disclosed in the U.S. patent application Ser. No. 09/805309.
In FIG. 7, this probe for micro-configuration measuring device is made of the above-mentioned touch sensor 10 and a fine motion mechanism 21 using a piezoelectric element (PZT), which are coupled together along the axis of oscillation of the stylus 12 and, as a whole, are attached to a movable support member 22.
Given such a structure, if an electrically alternating signal, which is characterized by the oscillation frequency and the oscillation voltage, are sent from the oscillator 3 to the oscillation means 17, then the stylus 12 starts oscillating in a resonating manner along its axis. If, in this state, the stylus tip 12A makes contact with the test piece W, the resonating state of the stylus 12 changes. Accordingly, by monitoring output from the detection means 18 indicating this change, the contact between the stylus tip 12A and the test piece W can be detected. Output from the detection means 18, which is designated as the detection signal DS1, is sent out to a detecting circuit 4. The detecting circuit 4 converts the detection signal DS1 into the detection signal DS2. The detection signal DS2 is filtered by a filter 51 to remove noises and sent out to a signal processing unit 62 as the detection signal DS3. The signal processing unit 62 computes a difference between the detection signal DS3 and a threshold which determines the measuring force and sends out the result to a controller 61. The controller 61 drives the fine motion mechanism 21 via a PZT driver 72 based on the result received. This system of controlling fine movement described so far allows the detection signal DS3 to be maintained constant with respect to any bumps and dins on the test piece W when the fine motion mechanism 21 and the test piece W are in relative motion for scanning.
In order to be successful in making non-destructive measurement on a test piece such as a silicon wafer, it is important how much the measuring force can be minimized. And, in order to minimize the measuring force, it is necessary that sensitivity of a touch sensor be boosted or the threshold be raised. What was conventionally attempted for the minimization of the measuring force is the boosting of the sensitivity of touch sensors through modification of their structure. However, such modification was not able to produce satisfactory results regarding the performance of micro-configuration measuring devices.